What if...? Computer Issues

[Cave Diver]

Well..... now I'm lost.

How do you guys know all this stuff??? It sure isn't in any of my dive manuals....(OW and AOW). Is this something you learn as dive master, instructor, or technical training?

I've had a lot of "interesting" dives. Some of this stuff I've learned the hard way. Some of it has come from hanging around people smarter and more experienced than me. Some of it has come from researching how not to find myself in those types of situations anymore.

 

[rhwestfall]

And I sure wish I could see you do it. It would make the second time in my life I have seen a wheel used outside of a classroom. Good to know they were not all wasted!

WASTED?

Funny thing is that I am not PADI certified, nor took a PADI class. I liked the format, liked the product, it had an excellent manual with it, and have used it since I became certified over 20 yeard ago (I learned in class on Navy Tables). I am amazed that I have never seen another one being used.....

Even though I am an engineer, I leave the math at my desk, and use the wheel.

 

[rhwestfall]

Well..... now I'm lost.

How do you guys know all this stuff??? It sure isn't in any of my dive manuals....(OW and AOW). Is this something you learn as dive master, instructor, or technical training?

Hunger to work it out.... Its the "enginerd" in me.....

(and no, I have no interest in Tech Diving)

 

[rhwestfall]

When you get back to your wheel, you can do another interesting exercise.

Do the same profile as a series of mini-dives with zero SI on the regular RDP. You will get the same results. All the wheel does differently is to add the the multi-level NDLs, add a bunch of rules about what are legal combinations of multilevels, and replaces the multiple rows of the SI credit table with just the one line.

The PADI NDL is 55 minutes at 60', but the MDL is only 46 minutes. The profile I described would exceed the MDL, and also would violate the rules of the PADI wheel because the next valid level shallower than 120' is 80'. And the next valid level above 80' is 60'. A dive computer that runs the DSAT model, such as most Oceanics, would be happy with the profile but would show low NDL. And it would seemingly take forever if you were to try and get the computer back into the green zone during your ascent and stops.

I just threw the profile above quickly together looking at a standard RDP.

If you want, you can do the wheel calculations on my flat table equivalent



Full size: ScubaBoard Gallery - "Flattened Wheel"

Yep, a multi level dive is essentially a series of "zero surface interval dives". My buddy used to work his tables that way, and we would compare notes at the end. Looks like a dive I wouldn't do.

 

[boulderjohn]

How do you guys know all this stuff??? It sure isn't in any of my dive manuals....(OW and AOW). Is this something you learn as dive master, instructor, or technical training?

Yes--all of the above, and out of personal curiosity.

A lot of it is in the manuals--just not the OW or AOW you have experienced. Much of this is covered in DM training. The PADI Encyclopedia of Recreational Diving has a lot of this. If you really want to get into decompression, read Mark Powell's Deco for Divers. If your math is way better than mine, read Bruce Weinke's Technical Diving in Depth.

 

[Charlie99]

Erik Baker has written some very clear, readable, yet complete articles on decompression. Index of /reference has a copy of the key ones.

I recommend the M-value one first. Then decolessons. Then NDL. Then the deep stop article would make sense.

The M-value article has the deco model limits for many models including the DSAT model on which PADI RDP and wheel are based. The NDL article has the basic formula, which can easily but put into a spreadsheet. The time at each depth/PG spot on the PADI table is the NDL for a reduced limit associated for that pressure group. I already knew that PADI RDP pressure groups and repetitive dive calculations are based on the 60 minute compartment. I was able to figure out the ppN2 or fsw N2 assigned to each PG with a bit of playing around with a spreadsheet. The other parameter that I had to diddle with to get the numbers in the table to agree with PADI was the value for water vapor + CO2 in the lungs. It appears PADI uses a conservative 1.6fsw, a bit less than USN and Buhlmann which are about 2 fsw.

I just dug up an old spreadsheet and verified that the bottom edge of PG Z is reached when the 60 minute compartment is 95% of the full M-value. PG A and B are oddball values of 12% and 21% of M-value. From C through Z it goes from 21% to 95% in steps of 3%.

As you can see in the attached spreadsheet, the PADI RDP can be replicated using just a few formulas, repeated multiple times. The attached spreadsheet also does the simplistic calculation of multplying times * ata the generate "surface equivalent minutes" for easy gas planning. At an SAC of 0.5, the 65cfm of gas in an AL80 between 3000 and 500 psi will provide you with 65/0.5 = 130 surface equivalent minutes of air.

If you look carefully at the SI credit table of the RDP, the section with zillions of numbers in tiny print, you can see that the time to go between any two PGs is independent of the starting PG. So all of those multiple rows and columns of numbers can be replaced by just one SI credit row, which starts in PG Z. You just need to do one addition to use the SI row. If you just finished a dive for 30 minutes at 80', you will be PG R. The 25 in SI row in that column means it would have taken 25 minutes to get down to R if you had started in Z. If your SI is 52 minutes, you add the 52 + 25 to find that you are 77 minutes down from Z. You entered H at 72, and won't get to G until 78, so your new pressure group after 52 minute SI is H. (When diving, you round upward in PG, when offgassing your round up in PG).

 

[rhwestfall]

Nicely done!

 

[boulderjohn]

The M-value article has the deco model limits for many models including the DSAT model on which PADI RDP and wheel are based. The NDL article has the basic formula, which can easily but put into a spreadsheet. The time at each depth/PG spot on the PADI table is the NDL for a reduced limit associated for that pressure group. I already knew that PADI RDP pressure groups and repetitive dive calculations are based on the 60 minute compartment. I was able to figure out the ppN2 or fsw N2 assigned to each PG with a bit of playing around with a spreadsheet. The other parameter that I had to diddle with to get the numbers in the table to agree with PADI was the value for water vapor + CO2 in the lungs. It appears PADI uses a conservative 1.6fsw, a bit less than USN and Buhlmann which are about 2 fsw.

I just dug up an old spreadsheet and verified that the bottom edge of PG Z is reached when the 60 minute compartment is 95% of the full M-value. PG A and B are oddball values of 12% and 21% of M-value. From C through Z it goes from 21% to 95% in steps of 3%.

As you can see in the attached spreadsheet, the PADI RDP can be replicated using just a few formulas, repeated multiple times. The attached spreadsheet also does the simplistic calculation of multplying times * ata the generate "surface equivalent minutes" for easy gas planning. At an SAC of 0.5, the 65cfm of gas in an AL80 between 3000 and 500 psi will provide you with 65/0.5 = 130 surface equivalent minutes of air.

Well..... now I'm lost.

I am sure that cleared everything right up for you.

 

[Charlie99]

Yep, a multi level dive is essentially a series of "zero surface interval dives". My buddy used to work his tables that way, and we would compare notes at the end. Looks like a dive I wouldn't do.

Riding the NDL like the profile I posted will rack up N2 loading much faster than a square profile NDL dive, and for a given air supply, it results in the highest possible loading while always staying inside NDL.

On a nice mellow warm water drift dive my SAC is 0.4 or so, so I have had personal experience with what riding the NDL (like the profile I posted) will do. Through trial and lots of errors, I've developed a pretty good feel for how long it will take may computer to move on back to the green zone.

On a second dive of the day to the wreck in Cozumel, I managed to get an Oceanic computer right to the edge of deco at 53', and after nearly 15 minutes of hang time at 15' to 10', it still hadn't gotten back to where I wanted it. I've approached the same sort of point several times after multilevel dives of 80' or 100', 70 minutes for a first dive out at Molokini, followed by a flat bottom dive to 60' range for a second dive.

 

[Charlie99]

I am sure that cleared everything right up for you.

The key is to read Erik Baker, not me.

And definitely don't try to read Bruce Wienke until you've gotten a very good understanding of multcompartment dissolved gas (aka neo-Haldanian, aka Buhlmann) models.